This invention relates generally to cranes and more specifically relates to cranes which rotate around a central axis, such as a jib crane.
The jib cranes used prior to the invention herein generally comprised a base plate for mounting the crane on the floor or foundation. A mast or pillar usually of a circular cross section, was welded or otherwise secured at the lower end to the base plate. Alternatively, the mast may be rigidly fastened in a concrete or structural steel foundation. A mast head bearing assembly was rotatably secured to the upper end of the mast. A boom or load beam was attached to the bearing assembly. The boom, unless otherwise desired, may be rotated in either direction for 360 degrees. The rotation of the boom and bearing assembly may be powered by air motor means, electrical motor means or hydraulic means, and in many instances may be manually controlled. The mast head bearing assembly included roller bearing means positioned at a fixed vertical height on the mast and revolved around the circumference of the mast, responsive to the rotation of the boom. Therefore, the prior jib crane consisted of three primary sections: the mast or pillar rigidly attached to the base plate (or secured in the foundation); the boom or load beam; and the rotatable mast head assembly. The three sections of the jib crane were generally operatively assembled in the field.
In order to achieve efficient operation of the crane it is required that the mast should be plumb and the boom level with respect to the ground and substantially perpendicular to the mast. When assembling the crane sections at the plant site, the boom was set at the proper level and the roller bearing means which revolves around the mast head was adjusted for proper frictional contact. In the event the mast became disaligned or initially was not plumb, shims or grouting was inserted at the base of the crane for obtaining the desired plumbness. If the mast is not plumb, the load or hoist could run away along the boom or the boom could rotate freely on its own. Usually, when the mast is plumb, the boom is also level with respect to ground. However, at times the boom of the prior jib cranes required level adjustment independent of the mast plumbness, or the bearing rollers needed adjustment due to wear or to vary the thrust forces acting on the mast from the loads carried by the boom. However, subsequent to the initial installation, the boom level and roller contact were very infrequently checked, if at all, due to the extreme inconvenience to make any corrective adjustments on the top of the mast. The subject invention overcomes this problem by rigidly securing the boom to the upper end of the mast, and any removable attachments or adjustments are conveniently placed within easy access from the floor area.
Previously, jib cranes having a rotating mast were also used. These jib cranes included a vertically extending mast having a lower end rotatably secured to a base plate and an upper end rigidly secured to a boom or load beam. The mast extended into a hollow pipe which was spaced from the inside surface thereof. Rollers were secured to the boom and in frictional contact with the pipe and revolved around the outside of the pipe as the mast and boom rotated. In the jib cranes described below in the Specification, the rollers are secured to the mast and in frictional contact with the pipe. In this manner, the rollers provide greater support for maintaining the plumbness of the mast. If the mast is not plumb, the position of the rollers could be adjusted for realigning the mast. Moreover, the subject invention, discloses means for precisely adjusting the position of the rollers and thereby controlling the transfer of the load forces from the boom to the mast substantially independent of the structural configuration of the mast.
In recent years, the crane business has become extremely competitive, as labor, materials and freight costs have sharply increased, and at the same time the availability of funds for captial expenditures have either decreased or high interest rates have discouraged excessive debt financing. In view of this situation, the subject invention is directed to the reduction of cost of materials without effecting performance and provides a jib crane which may utilize an "I" beam, wide flange beam, fabricated plate, channelled beam, or a tubing structure for the mast. Therefore, depending upon the design load specifications, a less expensive type structure could replace the more conventionally used tubing or cylindrical structure for the mast.